COS 34-1 - Host abundance amplifies parasitism in forest bumble bees

Abstract: North American bumble bee (Bombus spp.) populations are threatened by infectious disease. Temperate forests in the Pacific Northwest may provide important bumble bee habitat reservoirs, especially where management for timber production creates open-canopy forest. Bees are more abundant in these early seral patches due to increased light availability, non-native floral resources, and soil nesting opportunities, but the prevalence of infectious parasites in this habitat is unknown. Increased density of foraging bees combined with low diversity of floral resources may amplify disease transmission, while high diversity of bee hosts and flowers may dilute or negate an increase in transmission.

We determined parasite prevalence in forest bumble bees and are identifying factors influencing infection rates, including canopy type, local bee abundance and diversity, and local floral abundance and diversity. We randomly selected three stand age types (open, < 5 years; thinned, 50-70 years with experimental canopy thinning; and closed, >70 years) across the Oregon Coast and Cascades Ranges. We surveyed blooming vegetation and stand characteristics and used sterile technique to capture active pollinators during 2-4 visits per season (n = 182 stands, 2020-2022). Bumble bees were identified to species and screened for six common trypanosomatid and microsporidian parasites. Open-canopy stands have higher floral diversity, higher floral abundance, longer bloom periods, and higher proportions of nonnative floral species compared to closed-canopy stands. We corroborated that bumble bees are most diverse and abundant in open-canopy forest and found that bumble bee abundance in stands amplifies parasite prevalence. Our results illuminate the need for early seral forest management practices to enhance bee diversity in addition to abundance.